SIBO — Small Intestinal Bacterial Overgrowth

Under normal conditions the small intestine is colonized by far fewer bacteria than the large intestine — this is kept in check by several defense mechanisms: stomach acid, digestive enzymes, peristaltic movements that push food content forward (the so-called migrating motor complex), and the ileocecal valve, which is meant to prevent bacteria from the large intestine backing up. SIBO (small intestinal bacterial overgrowth) is a state in which one or several of these mechanisms fails, and the number of bacteria in the small intestine rises above what's typical for this segment, often with a shift in composition toward carbohydrate-fermenting species that, in excess, produce hydrogen or methane.

The clinical result is a cluster of symptoms most people associate more with irritable bowel syndrome (IBS) than with a distinct disease entity: bloating, a feeling of fullness after eating, excessive gas, abdominal pain, and — depending on the dominant gas — diarrhea (with hydrogen predominance) or constipation (with methane predominance, which is why it's now sometimes classified separately as IMO, intestinal methanogen overgrowth). This isn't a rare gastroenterological curiosity — meta-analyses show that a significant proportion of patients diagnosed with IBS test positive for SIBO, which makes this a practically important topic, not just an academic one.

At the same time, SIBO is today one of those gastroenterological diagnoses that has accumulated a fair amount of imprecise language — online it's often used to explain essentially any chronic digestive discomfort, frequently without any testing at all. It's worth separating two things: SIBO as a real, mechanistically described condition with concrete diagnostic criteria, and SIBO as a popular, sometimes overused label. This article sticks to the former meaning.

The theoretical gold standard for diagnosis remains a culture of fluid drawn directly from the small intestine during endoscopy, but in practice this is an invasive, costly method rarely performed routinely. The vast majority of SIBO diagnoses in clinical practice therefore rely on non-invasive breath tests — the patient drinks a glucose or lactulose solution, and hydrogen and methane concentrations in exhaled air are measured at regular intervals. A rise in these gases above a defined threshold, especially early in the test, suggests the presence of excess sugar-fermenting bacteria already in the small intestine, before it reaches the large intestine.

SIBO is rarely a disease in its own right — more often it's a consequence of another, underlying problem that weakens one of the natural mechanisms limiting bacterial numbers in the small intestine. The most common include intestinal motility disorders (e.g. in diabetes with autonomic neuropathy, systemic sclerosis, or after an acute gastrointestinal infection that damaged the migrating motor complex), anatomical changes after abdominal surgery creating intestinal "blind loops," ileocecal valve insufficiency, and reduced stomach acidity, which under normal conditions acts as a barrier limiting the number of bacteria that reach the small intestine further downstream.

This last point has direct clinical relevance, because reduced stomach acidity isn't only a consequence of disease — it can also be a side effect of treatment. Proton pump inhibitors (PPIs), among the most commonly prescribed drugs in the world and covered in more detail in our article on proton pump inhibitors, strongly and persistently reduce hydrochloric acid production, which with chronic use may weaken this natural barrier.

The mainstay of pharmacological SIBO treatment is antibiotics that act locally in the gastrointestinal tract, above all rifaximin — poorly absorbed from the digestive tract, which limits its systemic effects and side-effect profile compared with antibiotics absorbed into the bloodstream.

In clinical practice, however, treatment doesn't stop at the antibiotic itself. Because SIBO is usually the consequence of another underlying problem (motility disorders, anatomical changes, low stomach acidity), eradicating the bacteria alone without addressing the cause often leads to recurrence within a few months. A supportive — though not replacement — role can also be played by temporarily restricting fermentable carbohydrates in the diet (the low-FODMAP protocol, described in more detail in our article on the low-FODMAP diet), used more for symptom relief than as a standalone causal therapy.

SIBO is a mechanistically and clinically well-described condition, but it's also one of those topics where the label is easily overused without proper testing. Chronic bloating and digestive discomfort are worth treating as a signal to seek consultation, rather than immediately reaching for self-selected supplements "for SIBO" — especially since those exact same symptoms can have entirely different causes, and a mistargeted intervention only delays proper treatment.